Considerable attention has been paid for many years to the bottom-blown converter steel making process as a substitute for the conventional top-blown converter steel making process. This bottom-blown converter steel making process has some advantages over the top-blown converter steel making process. For example, since the agitation of molten iron is substantially enhanced as compared with the top-blown converter steel making process, the concentration of total iron value (T.Fe) in a slag is substantially reduced, and as a result, the iron yield of the steel making process is substantially increased.
It is, however, difficult to modify a top-blown converter into a special bottom-blown converter since the bottom-blow converter is substantially different in converter configuration and support structure from the top-blown converter, and it is economically undesirable to totally convert the a top-blown converter steel making process into a bottom-blown converter steel making process in an instant. Under these circumstances, an attempt has been made to somewhat modify the conventional top-blown converter into a converter of the type using bottom blowing in combination with top blowing, that is, a top-and-bottom blown converter which is a top-blown converter taking advantage of bottom blowing. With the assistance of bottom blowing, such a top-and-bottom blown converter steel making process is free from the disadvantages of the top-blown converter steel making process such as weak agitation which causes the concentration of iron value in a slag to increase to reduce the yield of iron, suppressed decarbonization reaction in a low-carbon region, and reduced dephosphorization and desulfurization. However, in the prior art top-and-bottom blown converter steel making process, it was recognized only that agitation could be enhanced simply by providing tuyeres at the bottom of the converter and blowing a gas therethrough, and no further consideration was given to the specific conditions for providing the maximum agitation effect. A sufficient agitation effect was not always achieved despite the addition of bottom blowing.
Bearing the above-mentioned problems in mind, the inventors carried out extensive experimental research to find a technique capable of providing the maximum agitation between slags and metal in the top-and-bottom blowing, or in other words, a technique for deriving the maximum bottom blowing effect when bottom blowing is used in combination in a top-blown converter, and found that agitation is not substantially enhanced as compared with single top blowing unless there is an interference between the action of an oxidizing gas blown from the top and the action of a gas blown from the bottom. Based on this finding, the inventors recognized that, to enhance the agitation effect, a specific relationship must be set between the position of bottom blowing tuyeres and a hot spot which is developed on the molten iron surface by an oxidizing gas injected through the top blowing lance, that is, a region of the molten iron surface which is at a high temperature due to the direct impingement of an oxidizing gas against the molten iron surface. As a result of further experimental research, the inventors have found that the agitation effect is substantially enhanced by setting the position of tuyeres in relation to the hot spot according the following conditions, thereby achieving the object this invention.